S Class

Beings in the S class are the highest class (the pinnacle of power in the YuYu Hakusho universe). They are so strong that even a "low" S class's strength could completely devastate Earth. In Demon World, there are only about 20 known. However, in the manga, Mukuro clearly labels Shigure as the weakest of her 77 elite, implying that they are all S class. A middle S class was noted to start to get close and that would achieve Upper S in less than 2 years. A single Lower S class Demon is strong enough to defeat a horde of five-hundred Upper A classes Demons by themselves with relatively little effort and are capable of destroying the Earth. Sensui's full power was said to be at this level, as was Yusuke's when he came back to life as a demon. Kurama and Hiei also reach S Class during the Three Kings saga. At the end of the manga and anime, Kuwabara could have reached the Lower S class or higher as well, but he stopped only at Middle to Upper A Class to focus on his studies. He does later ascend to S Class at the end of the manga and in Eizo Hakusho by the time of the second Demon World Tournament.

Being the strongest class and following the class system, they are completely humanoid with some nonhuman features like pointy ears, or horns, or none at all, appearing to be completely human. The general personality of the S-class is to be very reserved, not often letting their emotions decide things, but rather experience. This detaches them from humanity, making them similar to ascended beings, as seen with Yomi, Mukuro and Raizen, three S-classes that have managed to rule the Makai for centuries. As the finale drew nearer, S-classes that have human-like emotions like laid-back natures, grief and respect for their enemy who were all friends of Raizen, showing a complete contrast to the lowest classes that lived in fear, had no respect for one another, let alone their opponents and extreme arrogance. The S-class has displayed some of the most friendly and patient characters in the series, and likely implying a future that demons would one day become similar to this nature. In fact, the most carefree demon of all, Enki becomes the current ruler of all the Makai. Only three humans have reached this level, all Spirit Detectives, Sensui with his Sacred Energy, and Yusuke with his dual ancestry and kuwabara. Each S-class demon has immense power, earning the fears of the Spirit World, of being capable of destroying the planet, both in immense strength and powerful energy based abilities such as barriers, blasts, and dimension distortions. Unlike the other classes, S class cannot be attained by simple training. It has been showed or implied during the series that in order to achieve S class a Demon must undergo a profound change. This change can be caused by emotion, a special condition unique to the individual, specialized training, or a procedure performed upon them. In essence, a Demon must undergo a fundamental change to achieve S class. In the case of humans, S class power can only be achieved by attaining and mastering sacred energy as it has been stated that spirit energy has limits to its growth. (In the instance of Yusuke, it was his demonic transformation that allowed him to attain S Class. With Kuwabara, it's the fact that he gained specialized power due to his exposure to the Demon Gate)

Either Raizen, if he had continued to eat humans and retained his immense power, or Mukuro, if she remained enraged, would be the strongest characters in the series.

Three of the main cast had reached S class by the end of the manga, while the fourth, Kazuma Kuwabara, is both the weakest, and ironically the strongest full human on Earth after Sensui's death. In the anime, Kuwabara stopped in the middle A class.

Team Kurama is A/A+ in the anime, which is inaccurate because they held their own against powerful S class demons in the anime. In the manga they are all stronger than typical mid S-class demons.

S class demons have been shown to be the most emotional in the show. This points to their integrities being more human-like than any other class, in general.

In chapter 152, Kurama states that the S Class refers to any warrior the Spirit World is incapable of defeating and groups together low level underlings with god-like warriors. This also fits in line with the SDF commander Ohtake's statement of an S Class demon being "infinitely stronger than the A Class" as well as the god like difference between the Three Kings and their second in commands.

It was hinted that the gap in power is between Upper S class and Lower S class is larger than the gap between Lower S class and Lower E class characters.

It is heavily implied that Demons and Humans become ten to twenty times more powerful when they acesend from A Class to S Class, because it was stated by Shinobu Sensui that he was using less than 50% of his full power, but still had ten times more Spiritual energy than Yusuke, who was A class when he first fought against Sensui.